1. Field of the Invention
The present invention relates to a plated substrate which is made of a ceramic or glass material and has a surface covered by a plating layer or film. More particularly, the present invention is concerned with such plated ceramic or glass substrate which has sufficiently high bonding strength of the plating film to the substrate surface and exhibits excellent physical properties and durability.
2. Discussion of the Prior Art
A technique for forming an electrically conductive layer or film on a surface of a ceramic or glass substrate has been widely utilized in various fields of industry, for example, in the field of preparing substrates for electronic components. One of commonly practiced methods for forming such electrically conductive film is a so-called "thick-film forming" process such as screen-printing. Recently, however, there have been efforts made to apply a plating process to the formation of a conductive film on the ceramic or glass substrate, in an attempt to obtain the film having improved properties and facilitate the process steps for producing the plated substrate.
Nevertheless, there has not established any predictably reliable plating process which satisfies all of the requirements, such as power or ability of covering the desired surface area of the substrate, and physical properties, bonding strength or adhesive force and surface smoothness of the film formed on the substrate. For instance, an alumina substrate widely used as a typical ceramic substrate for hybrid integrated circuits (HIC) or other electronic components is metallized by a plating process, in place of a conventionally widely practiced thick-film method using a paste. In this case, the plating process provides better results in terms of the physical properties and surface smoothness of the plating film, but suffers from considerably low ability in terms of the force of adhesion of the film to the substrate surface.
To solve the above problem, the following methods have been proposed:
1) The surface of the alumina substrate is roughened by etching or other suitable means, as disclosed in published U.K. Patent Application GB-2 141 741A.
2) A porous under layer or undercoat is formed of a suitable material such as ceramic or glass, on the surface of the alumina substrate to be plated.
3) Where a copper plating is formed on the alumina substrate, a copper aluminate layer is formed between the copper plating film and the alumina substrate surface, for increasing the bonding strength of the copper plating by chemical bonding, as disclosed in Publication No. 58-93397 of unexamined Japanese Patent Application.
4) The substrate surface is covered by an undercoat formed of a metal such as Cr or Ti, which exhibits affinity to both the alumina substrate and the plating layer, as proposed in Publication No. 57-198696.
The first method 1) does not have sufficient stability in the bonding strength of the plating film, and tends to cause variation in the bonding strength of the plating film in the local areas of the substrate surface. The third method 3) suffers from some problems. For example, the copper plating film may be deteriorated due to oxygen trapped within the formed copper aluminate layer. Further, the copper aluminate may be reduced to copper, by means of tin (Sn) used as a solder on the substrate, during a durability test of a product which uses the substrate subjected to a soldering process. Thus, the third method 3) is not reliable in assuring sufficient bonding strength of the plating film.
In the second method 2), the roughness or undulation of the porous undercoat more or less reflects on the smoothness of the plating film. Namely, the relatively low smoothness of the plating film makes it difficult to accurately form a desired electrically conductive pattern by processing the film. Further, the reduced smoothness deteriorates the electrical properties of the conductive pattern, and lowers the ease of mounting components on the substrate (final product using the substrate). Moreover, since the porous undercoat is a ceramic or glass material insulating the alumina substrate and the conductive plating film, the heat dissipation and electrical properties of the substrate are adversely affected by the properties of the insulating undercoat which are inferior to those of the alumina substrate.
The fourth method 4) suffers from variation in the uniformity of an oxide film formed on the Cr or other metal undercoat. This variation results in lowering the ability of the applied plating film to cover the desired surface area of the substrate. That is, the oxide film may prevent the plating film from being uniformly formed on the entire area of the desired substrate surface to be plated. Further, the metal (e.g., Cr) used as the undercoat may diffuse into the plating film when the plated substrate is heat treated, if necessary.